They will be worn to the point of letting water in. I replace then every second tyre change.

I can't verify that.. just saying I do pack(smear a finger load) of grease, behind said seal, and that precludes any water ingress to anything behind it.. I only replaced an "outer seal" on one C10 I owned, as a preventative P/M once, as I had a chewed up seal on the speedo gear side, from a stupidly and rapidly found mis install..so I did both.(the seal on the gear side, and also the wheel r/h side) When proper bearings are installed, and a goober of grease is layered over them and outer seal in place, its a hands off, totally sealed thing, and can be cleaned, and reinstalled the same way for the life of the bike.. I do polish and clean the r/h "wheel" spacer o.d., that the seal contacts, every time, and have no issue after that.

Are you replacing just the seals every other change or bearings and seals? ThanksWayne

When replacing the seals for the first time I very carefully flip the bearing seal out with a tiny screwdriver, push a fingerful of grease into the ballrace, refit the bearing seal and fit new outer seals. I've helped friends do this on bikes which have come say 50,000 and there are water stains and slight rust on the bearings outer faces F&R.

With the wheel off, rotate the 'top hat' spacer/sleeve that the seal runs on and check how much tension, if any, the seal has.